The present invention pertains to satellite positioning system receivers, such as Global Positioning System (GPS) receivers. More particularly, the present invention relates to detecting that a particular satellite has provided a faulty measurement and then excluding the faulty measurement from further position solution activity.
Satellite positioning systems are well known for enabling users to precisely locate their positions on or near the Earth. Such systems are commonly used for navigation in many different applications, such as aviation, nautical travel, automobile travel, etc. One well-known satellite positioning system is the Global Positioning System (GPS). The GPS was developed by the United States Department of Defense under its NAVSTAR satellite program. The fully operational GPS includes 24 satellites dispersed approximately uniformly around six circular orbits with four satellites in each orbit. Three or more GPS satellites should be visible from most points on the earth""s surface, and access to three or more such satellites can be used to determine an observer""s position anywhere near the earth""s surface at all times. Each satellite carries atomic clocks to provide timing information for the signals transmitted by the satellites. Internal clock correction is provided for each satellite clock.
Four satellites at a minimum are needed to uniquely determine a user""s position in three dimensions and time. If only three satellites are visible, conventional GPS software solves for latitude, longitude and time. Time is nearly always necessary to be ascertained, and the altitude dimension can be constrained, e.g., assumed or provided.
Each GPS satellite transmits two spread-spectrum, L-band carrier signals. The xe2x80x9cL1xe2x80x9d signal has a frequency of 1575.42 MHz, and the xe2x80x9cL2xe2x80x9d signal has a frequency of 1227.6 MHz. These two frequencies are integral multiples of a base frequency of 1.023 MHz. The L1 signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, i.e., a coarse acquisition (C/A) code and a precision (P) code. The L2 signal from each satellite is BPSK modulated by only the P code.
Use of the PRN codes in a code multiple access scheme allows the sorting out of the GPS satellite signals that all share the same L1 and L2 frequencies. A signal transmitted by a particular GPS satellite is selected in a GPS receiver by generating and matching, or correlating, the corresponding, unique PRN code for that particular satellite. The PRN codes come from a short list, and each is stored in GPS receivers carried by ground observers.
The P-code is a relatively long, fine-grained code having an associated clock or xe2x80x9cchipxe2x80x9d rate of 10.23 MHz. The C/A-code allows rapid satellite signal acquisition and hand-over to the P-code and is a relatively short, coarser grained code, having a chip rate of 1.023 MHz. The C/A-code for any GPS satellite has a length of 1023 chips and thus repeats every millisecond. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code used for a given GPS satellite has a length of precisely one week (7.000 days) before this code portion repeats.
The GPS system is such that the C/A-code and P-code can be deliberately corrupted in one operational mode by random dithering that reduces position-fix accuracy. This mode is called Selective Availability (SA). A mode called Anti-Spoofing (AS) includes the transmission of an encrypted Y-code in place of the P-code. xe2x80x9cAuthorizedxe2x80x9d receivers can decode the Y-code, and such receivers can retain their accuracy in position fix determination during SA.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite and an almanac for all GPS satellites, with additional parameters providing corrections for ionospheric signal propagation delays suitable for single frequency receivers and for an offset time between satellite clock time and true GPS time.
Another satellite positioning system is the Global Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union and now maintained by the Russian Federation. GLONASS also uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. The methods for receiving and analyzing GLONASS signals for determining a user""s position are similar to those used for GPS.
Yet another satellite positioning system is the Galileo system, which is a satellite positioning system next to be introduced by European countries.
One problem associated with satellite positioning systems such as GPS, GLONASS and Galileo, is that an anomaly can occur in the signal transmitted from a satellite or in a measurement derived from a satellite signal in the receiver. The anomaly may be due to, for example, a problem within the satellite, a problem within the receiver, or interference in the signal path between the satellite in the receiver. Regardless of the cause of the problem, such an anomaly can result in an erroneous navigation solution in the receiver. Such an error can be intolerable in many applications, such as an aircraft landing assistance system, in which even a small error in the navigation solution can be critical. Therefore, some provision must be made to detect and exclude faulty measurements from the navigation solution, as they occur.
Certain GPS receivers are equipped with a Receiver Autonomous Integrity Monitoring (RAIM) system which performs this function. However, certain existing RAIM solutions are capable of handling only a limited number (e.g., eight) of satellite measurements for purposes of detecting and excluding faults. In addition, existing RAIM systems do not provide a high enough probability of isolating and excluding a fault for many applications and do not meet the stringent time to alarm requirements for non-precision approach and category I through category III landing. For example, the Federal Aviation Administration (FAA) requires a probability of exclusion of 0.999 or even higher for aircraft navigation. Note that existing GPS receivers generally attempt to detect and exclude a faulty measurement in a single combined step. This can result in the failure to isolate a faulty measurement or the incorrect exclusion of a non-faulty measurement, particularly for small errors. The reason for this is that the faulty measurement is not always the top candidate for exclusion.
The present invention includes a positioning system receiver capable of detecting and excluding a fault and a technique for carrying out such processes. The technique includes determining whether a fault detection operation is available, and attempting to detect a fault while the fault detection operation is available. The technique also includes determining whether a fault exclusion operation is available in response to a fault being detected, and attempting to exclude the fault if the fault exclusion operation is available.
Other features of the present invention will be apparent from the accompanying drawings and from the detailed description which follows.